The pulses to be measured are normally applied to a detection subsystem mainly comprising an amplifier called a “transimpedance” amplifier.
The main function of a transimpedance amplifier is to convert an input current lin into an output voltage Vout, with a transfer function of the type Vout/lin=−R1/(1+R1.C1.s) where s is the sampling variable or the Laplace variable, that is, a variable representing a frequency or a frequency component of the variable input current, R1 the value of a resistor and C1 the value of a capacitor. A transimpedance amplifier conventionally comprises a high-gain amplifier having for feedback, between its output and its input, a resistor of value R1 in parallel with a capacitor of value C1.
The transfer function of such a transimpedance amplifier is a function of the frequency, via the variable s. The cut-off frequency for which the output voltage loses three decibels compared to what it would be at low frequency is Fc=1/(2πR1.C1).
The transimpedance gain, that is, the amplitude of the voltage obtained for a given input current amplitude, is fixed by the value of R1, a value that is also involved in the cut-off frequency. The signal-to-noise ratio increases with the value of R1, but the bandwidth is then reduced accordingly. This configuration does not therefore allow for both a high bandwidth and a good signal-to-noise ratio.
The capacitor C1 is essential to the correct operation of the amplifier, because it ensures the stability of the configuration. It is chosen according to the desired gain and the desired frequency of use, and it is also chosen according to the input capacitance of the transimpedance amplifier (similar to a stray capacitance) and according to the absolute value G of the natural gain of the high-gain amplifier which forms the core of the transimpedance amplifier.
In a current pulse detection subsystem, the transimpedance amplifier can be followed by a voltage amplifier that is used to obtain a sufficient voltage level at the output if the transimpedance amplifier does not directly supply a sufficient level.
The object of the invention is to propose a pulse detection circuit with which to optimize the trade-off between the bandwidth and signal-to-noise ratio characteristics of the detection subsystem.